1. Field of the Invention
The present invention relates to a camera having a television mode. More particularly, the present invention relates to a camera having a normal mode for photographing a normal photographed object and a television mode suitable for photographing a picture image on the television screen such that both the modes can be selectively switched.
2. Description of the Related Art
A picture image displayed on a display screen of a cathode-ray tube of television is formed by dividing this image by 262.5 scanning lines. A first scanning operation is preformed for 1/60 second. At a second scanning time, the picture image is scanned by 262.5 scanning lines between the scanning lines in the first scanning operation. Thus, one picture image is formed on the television screen by using a total of 525 scanning lines and a total of scanning times 1/30 second.
In such a jump scanning system, when the picture image on the television screen is photographed, it is necessary to set a shutter time 1/60 second for completing the first scanning operation on the entire television screen, 1/30 second for completing the second scanning operation, or an integral multiple thereof. When the picture image is exposed at a shutter time except for 1/60 second, 1/30 second or the integral multiple thereof, there is a case in which the image has no central portion and partially has a bright band by overlapping the scanning lines with each other.
In a compact camera provided with a lens shutter of a program AE system, etc., when a picture image on the television screen is photographed, shutter speed and diaphragm are shifted by the change in brightness of the picture image on the television screen. Accordingly, when the picture image on the television screen is bright, the shutter speed is high so that the picture image on the televisiion screen cannot be suitably photographed.
Therefore, there is a camera having a function for switching a normal mode to a photographing mode for photographing the picture image on the television screen and such a camera is commerically sold.
When a mode change-over switch in this camera is switched to the photographing mode for photographing the picture image on the television screen, the shutter speed is set to a speed suitable for photographing the picture image on the television screen.
Japanese Patent Application Laying Open (KOKAI) No. 60-254027 shows a circuit for controlloing a program shutter. This control circuit has a sector for forming a lens aperture and a step motor rotated in normal and reverse directions to open and close this sector. The control circuit further has motor control data storing means for setting a step driving number corresponding to an exposure amount and an interpolating amount as data and storing the exposure amount in accordance with an address. The control circuit further has pulse generating circuit means for generating a step driving pulse and a closk pulse. The control circuit further has first counting means for presetting the step driving number before the sector is opened and the step motor is rotated in the reverse direction. The number of driving steps is counted down by the first counting means by the step driving pulse from the pulse generating means. The control circuit further has second counting means for presetting the interpolating amount data before the first counting means is counted up. A counting-down operation of the second counting means is performed by the clock pulse. The control circuit further has reverse rotation driving means for detecting a time point at which the second counting means is counted up and rotating the motor in the reverse direction. The control circuit further has a control section provided with motor driving means for driving the motor by a pulse from the pulse generating circuit means.
When a picture image on the television screen is generally photographed by a camera using a focal plane shutter, the operation of this camera is controlled by setting the shutter speed to a constant value and using the results of an exposure processing based on photometric information with respect to hte diaphragm of a lens.
In the camera having a function for switching the normal mode to the photographing mode for photographing a picture image on the television screen, only the shutter speed is set to a value suitable for photographing the picture image on the television screen when the television mode for photographing this picture image is set. However, an exposure value is constant irrespective of ISO information of a film and photometric information. Accordingly, the exposure value does not corresponed to the brightness of the picture image on the television screen. Therefore, there is a case in which the picture image is photographed at excessive and insufficient exposure amounts.
Further, in the control circuit of a program shutter proposed in Japanese Patent Application Laying Open (KOKAI) No. 60-254027, only a proportional relation between the diaphragm and the shutter can be controlled. Accordingly, when the picture image on the television screen is photographed, the operation of the shutter is controlled in only a state in which the ISO information and the photometric information are neglected and the exposure amount is constant. Therefore, similar to the above-mentioned camera, there is a case in which the picture image on the television screen cannot be photographed at a suitable exposure amount.
In the above camera using the focal plane shutter, the operations of the shutter and the diaphragm must be independently controlled and the structure of the camera is complicated, thereby increasing the cost of the camera.
In the case of a shutter having a plurality of diaphragm blades and having shutter and diaphragm functions, the shutter is completely opened in three steps after a trigger switch is turned on. When the number of steps from the beginning of an opening operation of the shutter to a complete opening state thereof at the normal photographing time is not set to be large at a designing time of the camera, a quantization error in exposing control is increased so that exposure dispersion is increased. To increase the number of steps, it is necessary to increase the number of rotations of the step motor. However, torque of the step motor is decreased as the rotational speed of the step motor is increased. Accordingly, there are many restrictions with respect to the design of mechanical portions in the camera.
Further, in a program shutter described in Japanese Patent Application Laying Open (KOKAI) No. 61-47935, a diaphragm value is necessarily determined when a shutter time suitable for photographing a picture image on the television screen is selected. Accordingly, the operation of the shutter is controlled in only a state in which film sensitivity information and photometric information are neglected and the exposure amount is constant. Therefore, similar to the above-mentioned camera, there is a case in which it is impossible to photograph the picture image on the television screen at a suitable exposure value.